Method and apparatus for modifying limit and protection software in a synchronous generator exciter to match the capability of the turbine-generator

ABSTRACT

A method and apparatus for compensating, consistent with variations in cooling conditions, protection and limit functions of a generator. Limits of the capability curve of the generator, including the overexcited region, underexcited region and region limited by stator current heating, are modified in response to changes in coolant pressure or temperature depending on the type of generator being compensated. Feedback signals and setpoints being provided to fixed protection and limit functions are intercepted and modified in accordance with the relationship between generator rated values and generator performance in view of altered coolant conditions. Protection and limit functions are automatically coordinated. As a result, overall generator performance is improved.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. Ser. No. 09/369,181, filed Aug.6, 1999.

BACKGROUND

Exciters designed for operation with synchronous generators havetraditionally been required to protect the generator field by limitingoperation of the generator in the overexcited region of a generator'scapability curve which is restricted by field heating. In theunderexcited region of the generator capability curve, operation must belimited because of stator end turn heating and stator laminationover-voltage effects. Operation between rated KVA at rated lagging powerfactor and rated KVA at unity power factor must be limited because ofoverheating caused by excessive stator current. While conventional fixedhardware or software algorithms used to implement this functionalitysuch as General Electric's GENERREX-CPS, GENERREX-PPS, SHUNT-SCR,ALTERREX, STATIC BUS FED EXCITATION and SILCO 5 control implementations,can properly protect the generator these prior art implementations donot have any capability of being responsive to coolant conditions asthey deteriorate from nominal. A further problem is that prior artlimiter implementations do not account for improved coolant conditionsand the possible increase in generator rating associated with suchimproved coolant conditions. Indeed, “ambient following” combustionturbine applications are particularly sensitive to changes in coolantconditions as the output power capability thereof varies significantlywith coolant conditions.

It is thus seen to be desirable to coordinate the limit and protectionfunctions of a generator exciter as a function of coolant conditions toprovide improved overall generator performance.

SUMMARY OF THE PREFERRED EMBODIMENTS

The apparatus and method described herein accurately protect thegenerator as coolant conditions vary from nominal in the areas limitedby (1) field heating, (2) stator current heating and (3) underexcitedcapability while maintaining full output capability from the generator.In a preferred embodiment, appropriate algorithms implemented inhardware or software are automatically selected to optimize thecompensation for either hydrogen cooled or air cooled generators in thearea of the capability curve characterized by field current or statorcurrent heating while a jumper, or other suitable switching device, isimplemented to select the proper compensation in the underexcited area.The advantages of the preferred embodiment include implementation ofvery accurate compensation of limiters with automatic coordination ofthe limiters and protection algorithms, whereby a more efficient use ofa generator across varying cooling conditions is realized.

Since both limit and protection algorithms can be supplied in an exciterof a generator, it is, in accordance with the preferred embodiment,desirable that the limit and protection functions be coordinated andmaintain coordination as coolant conditions change. Thus, describedherein, is a very accurate apparatus and method for coordination of thelimit and protection functions, as a function of coolant conditions,permitting full output capability from a turbine generator whilemaintaining very accurate protection functions. As an added benefit, acoolant compensated stator current limit function can be added foroperation in the area of the capability curve from rated KVA at ratedlagging power factor to operation at rated KVA and unity power factor.

More specifically, generators have different ratings based on thecoolant conditions at the existing operating point. As noted above,prior art exciter implementations use fixed limit and protectionhardware or software algorithms which either do not offer protection atother than rated coolant conditions or do not permit generator operationto increase as coolant conditions improve to obtain full capabilityoperation of the generator. This is especially important for ambientfollowing combustion turbine applications, as previously explained.Thus, the methodology described herein implements fixed parameter limitsand protection algorithms yet provides full limit and protectioncompensation keyed either to temperature or hydrogen pressure (dependingon the type of cooling system) to match the capability of the exciter tothe generator rating. And, as a consequence, there is provided automaticcoordination between the limit and protection algorithms.

In accordance with the preferred embodiment, a two part characteristicis used to implement the under excited limit (UEL). Utilizing such a twopart characteristic for the under excited limit has the added benefit ofproviding a compensating signal for operation in the area of thegenerator capability curve determined by stator current heating orstator KVA. This permits the use of a fixed parameter stator currentlimit that is compensated for coolant conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical air-cooled generator capability curve.

FIG. 2 illustrates a typical hydrogen-cooled generator capability curve.

FIG. 3 illustrates a typical European manufacturer's air-cooledgenerator capability curve.

FIG. 4 illustrates a block diagram showing fixed limiters and protectionwith coolant compensation in accordance with a preferred embodiment.

FIG. 5 illustrates a hydrogen pressure and ambient temperature inputsensor in accordance with a preferred embodiment.

FIG. 6 illustrates an underexcited limiting and stator current limiterarrangement for compensation of hydrogen pressure and ambienttemperature in accordance with a preferred embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a typical reactive capability curve for an air-cooledgenerator. The generator rating at rated inlet air temperature is givenby the curve through points A B C and D. Two other capability curves areshown in FIG. 1, demonstrating the capability of the generator whenoperating at temperatures that differ from rated. The curve from A to Bis limited by field heating in the generator and results in a maximumfield current value that is permitted for steady state operation. Theover excited limit (OEL) and associated protection is used to limitoperation of the generator in this section of the capability curve.Curve B C is limited by stator current heating. A stator current limitis used to restrict operation of the generator when the generator outputfalls within this region of the capability curve. Curve C D reflects thecapability of the generator to operate in an area of the capabilitycurve limited by end iron heating and stator lamination over-voltage.Curves E F G D and H J K D illustrate the generator capability at 50Degrees C. and −18 degrees C. respectively.

FIG. 2 illustrates a typical hydrogen-cooled generator capability curve.In the case of hydrogen-cooled generators, generator rating occurs whenthe generator is at the maximum hydrogen pressure and the generatorrating decreases as hydrogen pressure is reduced. The rated hydrogenpressure capability curve is denoted as curve ABCD in FIG. 2 and thecurve for the lowest hydrogen pressure is denoted as curve EFGD. Thecapability curve limitations are similar to the curves in FIG. 1. Thatis, in FIG. 2 curve A B is limited by field heating, curve B C islimited by stator current heating, and end iron heating and statorlamination over-voltages limit curve C D.

FIG. 3 illustrates a typical air-cooled generator capability curve froma European manufacturer. Instead of the common US practice of showingthe zero power axis on the left side of the graph, the standard Europeanpractice, as illustrated in FIG. 3, is to show the zero power point onthe right. The capability curve limitations are similar to the curves inFIG. 1. Curve A B is limited by field heating. Curve B C is limited bystator current heating. End iron heating and stator laminationover-voltages limit Curve C D. The curves from C to D, G to D and K to Dillustrate a difference between these generators and the standard U.S.air-cooled generators. None of these curves shows any variation withcoolant temperature.

In accordance with the preferred embodiments discussed herein, there isprovided limiters and protection apparatus and methods (hereinafter“algorithms” or “functions”) for exciters for synchronous generators.Specifically, the limiters disclosed herein, through regulator action,limit operation of the exciter and generator to acceptable operatingareas of a generator's capability curve. Protection algorithms, on theother hand, preferably do not have any regulator functions but insteadgenerate trips in response to operation of the exciter and generatoroutside the capability curve of the generator. As described herein thereis provided limiter algorithms that actively force the exciter tooperate within the capability curve in steady state conditions and canbe compensated for coolant conditions.

Further, the preferred embodiment permits transient penetration of acoolant compensated forbidden area of the capability curve for systemevents, thus maximizing the contribution of the generator to systemstability. By implementing similar algorithms for the limiter andprotection functions it is possible to effectively coordinate betweenthe limiter and protection functions. Consequently, it is possible, inthe instance of the OEL, to ensure that the limiter, if it is working,will limit operation of the generator to the coolant compensated area ofthe generator capability curve limited by generator field heating. Thisensures that the protection does not trip undesirably, therebymaintaining the generator on-line for longer periods and during periodsduring which conventionally would result in a tripping situation. Inother words, the limiter algorithms control the amount of currentflowing in the field winding and does so in coordination with protectionalgorithms.

Specifically, in the field circuit of a generator, the total temperatureof the field winding is an important criterion. The total field windingtemperature is affected by the field current in the winding and thecondition and temperature of the coolant. Generally, generators used insteam turbine applications tend to operate at a fixed cold gastemperature and do not follow ambient temperature variations. In thisinstance, the gas pressure varies instead and the capability of thegenerator decreases with lower gas pressures. The relationship betweenthe generator field current and gas pressure in a gas cooled generatorcan be defined as a log-log function which is well known to thoseskilled in the art. Specifically, if the present gas absolute pressureis (PGAS+14.7) lbs/in² (hereinafter units of pressure and temperature,e.g., degrees C, are to be understood, although not expressly stated)then the maximum allowable field current at that pressure is

IF@PGAS=IF _(—) RAT*((PGAS+14.7/(PGAS _(—) RATED+14.7)){circumflex over( )}(H2FEXP/10000)  (1)

IF_RAT (rated field current) and the term H2FEXP can be determined fromgenerator data for a particular application. IF@PGAS is the fieldcurrent rating specified by the manufacturer at a particular gaspressure. H2FEXP is an exponent that can be estimated from the fieldcurrent and hydrogen pressure data supplied by the generatormanufacturer and the “10000” divisor in Equation (1) results from thespecific implementation used to relate digital counts to engineeringunits. Generally, the term (H2FEXP/10000) varies between 0.4 and 0.5 fortypical hydrogen cooled generators. The preferred embodiment maximizesthe accuracy of the approximation illustrated in Equation (1) forraising the ratio of the two absolute temperature signals to a powerthat varies between 0.4 and 0.5.

In this regard, both sides of Equation (1) can be divided by IF_RATwhich yields, on the right hand side of such a modified Equation (1), amultiplier which is used to modify (a) feedback signals and/or (b) limitreference setpoints. This permits implementation of protection functionsand limiters that are capable of being compensated for coolant changeswhile still using fixed algorithms for the protection and limiterfunctions.

FIG. 4 illustrates a block diagram of the preferred implementation of acoolant compensated under excited limit (UEL), OEL (both limiter andprotection) and stator current limit. Only the blocks pertinent to theimplementation of coolant compensation are described in detail. Theblock labeled UELIN, block 400, is the input block for the UEL functionand provides the functions of normalizing by 1/VT{circumflex over ( )}2,low pass filtering, enabling and an absolute value circuit. Inputs powerP and terminal voltage V_(T) are provided to block 400. The table lookup, block 405, is an arbitrary function generator, used to implement thecapability curve from point C to D on the rated coolant capabilitycurve. The block labeled UELOP, block 415, is used to remove the1/VT{circumflex over ( )}2 normalization from the coolant compensatedVar reference, the output from the UELHT block, block 410. The blockE1PFV, block 425, uses the signal HTSMAXVA (explained below) to modifythe stator current limit reference so that raise and lower commands areissued to an automatic voltage regulator (AVR) (not shown) to limitstator current consistent with the coolant conditions. Ratio block 430is used to modify the OEL field current regulator reference. Ratio block435 is used to modify the field current feedback signal to the limit andprotection algorithms.

FIG. 5 illustrates a preferred technique to develop a multiplier(HTS_BIAS) to compensate the over excited limiter (OEL) and protectionalgorithms for hydrogen cooled generators. The term HTS@IN is the outputsignal of a hydrogen pressure transducer associated with the coolingsystem of the generator (not shown) and generally supplied by thegenerator manufacturer. This signal is passed through low pass filter500 to remove undesirable AC components. Further, the term HTSOFS(hydrogen temperature sensor offset) is selected to remove, via summingblock 515, any offset resulting from the use of, for example, a 4-20 macurrent signal for transmitting hydrogen pressure (as embodied inHTS@IN). In the case of hydrogen cooled generators, the current signalis indicative of coolant conditions, as discussed previously herein. Itis common practice to use an offset transducer (4-20 ma) so that a zerosignal level is not permitted and signal levels less than, for instance2 mA., can be interpreted as an open wire.

The term HTSSCL (hydrogen temperature scalar) in scaling block 520provides the ability to scale the absolute hydrogen pressure signal for10000 counts at rated absolute pressure. The use of the 10000-countdivisor is arbitrary and any suitable value can be used depending onimplementation considerations. The term HTSFAL (hydrogen temperaturesensor failure), fed to comparator 545, permits detection of a failurein the input signal (HTS@IN) from the transducer that results from,e.g., an open wire. This is used to generate a signal HTSENBAD (hydrogentemperature sensor bad) for diagnostic purposes (as desired) andpreferably to switch, via switch 530, the input signal to a value HTSNOM(hydrogen temperature sensor nominal) so that no compensation is appliedwhen the sensor or transducer fails. HTSSENSE provides a diagnosticsignal representative of the output of switch 530, Block 525 provides anupper limit (HTSULM) and lower limit (HTSLLM) for the value exitingscaling block 520.

TMPRAT (rated temperature) is entered as a zero in this instanceresulting in the automatic selection of the uppermost path, namelyfunction block 550, shown on the right hand side of FIG. 5. This resultsin the generation of HTS_BIAS (the multiplier) for compensating, as afunction of hydrogen absolute pressure, the OEL and protection. In thepreferred implementation, HTS_BIAS is scaled, in multiplier block 575,for 10000 counts equal to unity ratio. The preferred technique is to usea ratio block (FIG. 4) to multiply the feedback signal (IFG) to aninverse time protection algorithm by 10000/HTS_BIAS and the reference tothe OEL (FCR_REF) by HTS_BIAS/10000 (ratio blocks 435 and 430respectively) thereby providing compensated reference values to thefixed limit and protection algorithms.

In contrast to steam powered generator applications, generators poweredby combustion turbines operate in an ambient following mode at a fixedgas pressure (which would be zero for an air-cooled machine). Therelationship between field current and gas temperature is based onquadratic interpolation functions that are well known by those skilledin the industry. For gas temperatures less than rated, the equation forfield current is given by

IF@TLOW=SQRT((IF _(—) RAT{circumflex over ( )}2+TMPSFB*(TGAS−TGAS _(—)RATED))  (2)

For gas temperatures greater than rated, a different interpolationfunction is more appropriate and is given by

IF@THIGH=SQRT((IF _(—) RAT{circumflex over ( )}2+TMPSFA*(TGAS−TGAS _(—)RATED))  (3)

The coefficients TMPSFA and TMPSFB can be determined, using Equation (2)or (3), from generator data supplied by the generator manufacturerrelating field current at a given coolant temperature to field currentat rated temperature. Similar to hydrogen pressure compensation, bothsides of Equation (2) and (3) can be divided by IF_RAT to yield amultiplier that can be used to modify feedback signals and/or modifylimit setpoints so that fixed protection and limiter algorithms can beused. In the preferred implementation this multiplier is again calledHTS_BIAS and is scaled, in multiplier block 575, 10000 counts for unityratio. The preferred implementation automatically selects betweenhydrogen pressure or temperature compensation based on the entry ofrated temperature (TMPRAT). That is, a value of 0 for TMPRAT results inselecting hydrogen pressure compensation while a non-zero value forTMPRAT selects temperature compensation, via switch 570. As withhydrogen pressure, the preferred implementation utilizes an offsetcurrent signal so that an open wire condition can be easily detected. Inthe case of temperature, a non-zero input is required for TMPRAT so thata signal WRK14, which represents the difference between the actualtemperature and rated temperature, can be calculated by summing block540. Because TMPRAT has a value other than zero, the lower path, namely,function block 555, on the upper right hand side of FIG. 5 is selected,via switch 570. This results in a correct implementation of Equation (2)or (3) when both sides of Equation (2) or (3) are divided by IF_RAT withIF_RAT replaced by a count value of 5000 counts. 5000 counts is usedbecause equipment is typically scaled for 5000 counts at AFFL (whereAFFL is the field current required at full load, rated power factor andat rated temperature). This count value, however, is arbitrary and canbe changed depending on implementation circumstances. The preferredtechnique is to use a ratio block to multiply the feedback signal to theinverse time protection algorithm by 10000/HTS_BIAS and the reference tothe OEL (FCR_REF) by HTS_BIAS/10000 as shown in FIG. 4, blocks 435, 430.That is, the feedback signal is intercepted and modified in the desiredmanner.

The Under Excited Limit (UEL) preferably should account for end ironheating, stator lamination over voltage and steady state stabilityconcerns in arriving at a setting which will adequately protect thegenerator and maintain system stability.

FIG. 1 illustrates a typical generator capability curve for anair-cooled combustion turbine driven generator. In a preferredembodiment there is provided a method and apparatus for modifying theUEL in the area of the generator capability curve limited by end ironheating and stator lamination over voltage effects by modifying afunction generator. The function generator is used to approximate theunder excited capability of the generator (shown as curve CD in FIG. 1).The generator underexcited capability at zero real power is a singlevalue and then varies at other power levels based on either coolanttemperature or coolant pressure. A preferred UEL coolant compensationimplementation is illustrated in FIG. 6. The function generator outputis shown as UEH@IN in FIG. 6 and is used as an input to summing block650 of UELHT block 410. The preferred implementation then modifies thefunction generator output by a function that varies with the coolantcondition (5000-HITSMAXVA)*(UEHTC1 or UEHTC2) (where HTSMAXVA varieswith coolant condition) times either the real power or real powersquared, as depicted by function blocks 615, 620, 625, 630, 670 and 675.Generally, best results are obtained using real power forhydrogen-cooled generators and real power squared for air-cooledgenerators. Jumper or switch block 695 is used to select either realpower with a setting of 0 or real power squared with a setting of 1.UEHTC1 is a coolant coefficient used to modify the function generatoroutput as a function of hydrogen pressure or ambient temperature (forambient temperature greater than rated) and UEHTC2 is a coolantcoefficient used to modify the function generator output for ambienttemperature less than rated. From the generator capability curvesillustrated in FIG. 1, the function generator, block 405, is selected toapproximate the curve DCL where the curve from C to L is just anextension of the rated temperature curve from rated KVA at 0.95 powerfactor to a real power point equal to 120% of the rated KVA. For thecase shown in FIG. 1, margin e.g. 10%, is added to the capability curveusing UEHTC4 and results in the dashed curve MNP. To complete thedesired UEL characteristic, a straight-line is added through points Nand Q and a “max value” selection is made using block 660 of FIG. 6. Theslope of the line through N and Q in FIG. 1 is calculated as(sin(arccos(0.95)−margin))/(1−0.95). UEHTC3 is then found by multiplyingthe slope of the line through N and Q by 1000. For the capability curvesin FIG. 1, UEHJMP.0, block 695, is set to a 1 for a second orderinterpolation and UEHTC1 is found using the following formula

UEHTC1=5000*VAR1*GEN _(—) MVA{circumflex over ( )}2/(WATT1{circumflexover ( )}2*(GEN _(—) MVA−MVA@WATT1))  (4)

Where VAR1 is the difference in MVARS between the high temperaturegenerator capability curve at point G and the rated temperaturecapability curve DCL. GEN_MVA is the rating of the generator at ratedtemperature, WATT1 is the rating in MWATTS at point G, and MVA@WATT1 isthe MVA rating of the generator at point G. Similarly, UEHTC2 is foundusing data from the low temperature capability curve DK and is given bythe equation

UEHTC2=5000*−VAR2*GEN _(—) MVA{circumflex over ( )}2/(WATT2{circumflexover ( )}2*(GEN _(—) MVA−MVA@WATT2))  (4)

Where VAR2 is the difference in MVARS between point K and the ratedgenerator capability curve DCL, WATT2 is the MWATT rating of point K andMVA@WATT2 is the MVAR rating of point K.

For the case of a hydrogen-cooled generator, UEHJMP.0, block 695, is setequal to 0 for a linear interpolation with power and UEHTC1 is given bythe equation

UEHTC1=5000*(VAR1*GEN _(—) MVA)/(WATT1*(GEN _(—) MVA−MAXVA))  (5)

Where VAR1 is the difference in MVARS between the curve at lowesthydrogen pressure and most leading power factor for which constantstator MVA is maintained and the curve at rated hydrogen pressure. WATT1is the rating in MWATTS of the generator at the lowest hydrogen pressureand most leading power factor for which constant stator MVA ismaintained. MAXVA is the MVA rating of the generator at the lowesthydrogen pressure. Because hydrogen pressure is assumed to be at ratedor less, UEHTC2, block 625, is entered as zero. The margin is entered inUEHTC4, block 655, and the slope in UEHTC3, block 685, as was done forthe air-cooled generator.

FIG. 5 illustrates the preferred method for obtaining a value for theterm HTSMAXVA. The generator KVA varies as a logarithmic function ofabsolute hydrogen pressure for hydrogen cooled generators, but isdefined by a quadratic interpolation function for air cooled generators.This is similar to the variation of field current with coolantconditions, but different exponential coefficients for hydrogen cooledgenerators and quadratic interpolation functions for air cooledgenerators are appropriate. These functions are shown in the bottom halfon the right side of FIG. 5 and result in the generation of signalHITSMAXVA that is scaled for 5000 counts in block 585 at a ratio ofMAXKVA/GEN_KVA of unity. As in the case of field current, the preferredembodiment of HTSMAXVA automatically selects hydrogen pressure if TMPRATis entered as zero, block 580. The value to use for air-cooledgenerators is selected based on whether temperature is greater than orequal to rated or less than rated, blocks 560, 565 and 580.

To properly normalize HTSMAXVA to account for variations in generatorterminal voltage requires that HTSMAXVA be divided by per unit generatorvoltage squared. This is accomplished in FIG. 5 using blocks 505, 510,535, 536, 537, 538, 539, and 541. Blocks 537, 538, 539 and 541 comprisea digital low pass filter with a filter time constant, UELVLP entered inblock 538. Filtered generator voltage is used to remove any interactionswith local generator dynamics. Ratio blocks 505 and 510 are used todivide the output of block 585 by per unit generator voltage. One of theinputs to multiplier block 505 is 20000 resulting in a multiplication ofthe signal from block 585 by 20000. Block 510 is a ratio block thatdivides the input at x, 20000 times the output from block 585, by theinput at y, filtered generator voltage. The value 20000 is used in thisparticular implementation for rated generator voltage. Any suitablevalue could be used. Blocks 535 and 536 then divide block585-output/per-unit generator voltage by per unit generator voltageresulting in the desired signal at the output of block 536 beingproperly compensated by (1/(per unit generator voltage squared)).

Typically, generators have the capability of operating underexcited atrated KVA to some leading power factor, which may or may not be the sameas rated lagging power factor. This is illustrated by point C on FIG. 1.When this power factor point is reached there is a discontinuity in thecurve that determines the under excited limit. The under excited limitfor real power levels that exceed the discontinuity is determined by theconstant KVA or stator current curve which is a straight line withconstant slope through points N and Q of FIG. 1. If one were to try tomodify the output of the function generator UEH@IN of FIG. 4 to accountfor the discontinuity, it would be difficult to obtain a singlecompensation term because the slopes are very different in the twoareas, as can be readily seen in FIG. 1. In the preferred implementationa high value gate 660 (FIG. 6) is used to select between the compensatedfunction generator output from summing block 655 (discussed below) andthe output from the compensated constant KVA function, divider block690. While it would be possible to generate the constant KVA functionexactly, the preferred implementation utilizes a straight-lineapproximation to the constant KVA function. This is done to eliminatethe possibility of infinite (or very large) slope (delta Vars/deltaWatts) in the constant KVA function resulting in infinite or very highgain which would adversely impact the stability of the under excitedlimit (UEL). This is accomplished in the preferred implementation asshown in FIG. 6.

Specifically, a signal proportional to the maximum KVA at a givencoolant condition (HTSMAXVA) is subtracted, in summing block 680 fromthe real power signal (UELWATTS). The desired slope (UEHTC3/1000) thenmultiplies this difference, via blocks 685 and 690. The result is astraight line limiter characteristic that varies with generator coolantconditions having a fixed slope that provides limiter operation in thegenerator constant KVA section of the generator capability curve (curveNQ). Using a fixed slope avoids the potential for instability caused byinfinite or very large slope in the delta Vars/delta Watts'scharacteristic if the actual constant KVA curve were implemented.UEHTC4, input to summing block 655, provides for margin to be added tothe function generator part of the underexcited capability curve. Thispermits the user to enter points from the actual capability curve (DC ofFIG. 1) and then include the margin as a separate input. Thestraight-line limit passes through points N&Q.

FIG. 2 illustrates a typical generator capability curve for ahydrogen-cooled generator. The rated capability curve for ahydrogen-cooled generator is shown as curve ABCD in FIG. 2 and occurs atthe maximum generator hydrogen pressure. Ratings at other than ratedpressure result in less generator capability. As in the case ofair-cooled generators, curve DCL is the underexcited capability of thegenerator and is set using the arbitrary function generator, block 405,in FIG. 4. Margin is added to the curve using UEHTC4, block 655 of FIG.6, resulting in curve MNP in FIG. 2. For hydrogen cooled generators,UEHJMP.0, block 695, is set to a zero resulting in a linearinterpolation with UELWATTS. UEHTC1, block 620, is set to

UEHTC1=5000*VAR1*GEN _(—) MVA/(WATT1*(GEN _(—) MVA−MAX _(—) MVA)  (6)

Where VAR1 and WATT1 are shown on FIG. 2 and MAX_MVA is the MVA ratingof the generator at point G of FIG. 2. Because hydrogen pressure isassumed to always decrease from rated, UEHTC2, block 625, is set tozero. As with the air-cooled generator, the UEL characteristic isdetermined by a maximum value select circuit, block 660 in FIG. 6,between the curve MGP and a straight line segment through NQ of FIG. 2.Blocks 680, 685, and 690 of FIG. 6 are used to generate a straight-linesegment with a slope determined by UEHTC3, block 685. Block 690 is usedto normalize the slope, 1000 counts equals unity slope. UEHTC5, block665, is used to limit the UEL reference between −32767 counts and UEHTC5counts.

Utilizing a two-part characteristic for the UEL curve provides theability to approximate the generator under excited capability curve whenthere is no variation with coolant conditions for generator outputlevels less than rated KVA. This is a common practice, particularly withEuropean generator manufacturers, and results in a single underexcitedcapability curve up to the constant KVA portion of the capability curve.This is shown as FIG. 3. At generator KVA output levels less than ratedfor the given coolant condition, the UEL characteristic is determined bythe function generator output, 600 in FIG. 6. The function generatoroutput will not be modified as a function of coolant if coolantcoefficients UEHTC1, block 620, and UEHTC2, block 625, are set equal tozero. All the variation with coolant condition occurs in the constantKVA region and can be accommodated by selecting the proper slope usingUEHTC3, block 685. The middle temperature curve is selected as the ratedtemperature curve. At point C of FIG. 2, add the desired margin to theMVAR rating in per unit (sin(arccos(0.95)−0.1). The slope from FIG. 2 isthen found from

Slope=(sin(arccos(0.95))−0.1)/(1.24−(0.95*1.24)))  (7)

UEHTC3, block 685, is found by multiplying the slope by 1000 and UEHTC4,block 655, is found by multiplying the margin in per unit by 5000. CurveMNP of FIG. 3 illustrates the capability of the generator in theunderexcited region with the appropriate margin added by UEHTC4. Thestraight line through points NQ illustrates the capability in theconstant stator MVA part of the capability curve. The slope of the lineis determined by UEHTC3, block 685, and normalized by block 690.Variation with ambient temperature is accomplished by varying HTSMAXVA,block 565 of FIG. 5. The UEL characteristic is determined by a maxselection of the function generator with margin, block 655, and thecompensated straight line slope, block 690, using a max select function,block 660.

In the overexcited part of the capability curve (curves AB, EF, and HJof FIG. 1), field heating is the limiting parameter and a technique hasbeen described for implementing an over excitation limiter (OEL) whichcan be compensated for coolant conditions to enable the exciter toadequately protect the generator field and yet provide transient forcingcapability for system events. In the underexcited part of the capabilitycurve, it has been shown that the under excited limit (UEL) can becompensated for variations in coolant conditions using a max valueselection, via block 660 in FIG. 6, of a two part characteristic. Afunction generator (FIG. 4) which can be modified to reflect coolantconditions is used to implement the limit characteristic in the area ofthe capability curve less than rated generator KVA at the particularcoolant condition. A straight-line limit in the constant KVA section canbe compensated for coolant conditions and has been shown to provide avery good approximation to the underexcited capability curve in the arealimited by rated KVA at the given coolant condition. The straight-lineapproximation decreases the potential for instability in the UEL causedby the very high slope of the actual capability curve, especially aroundunity power factor. The area of the capability curve not covered byeither the OEL or UEL is from rated KVA at rated lagging power factor torated KVA at unity power factor (curve BQ in FIG. 1). This is the areaof the generator capability curve that is limited by the turbine outputand by stator current heating. The exciter can only transiently affectreal power requiring the exciter to reduce generator reactive outputpower as a means of reducing generator output KVA or generator statorcurrent. Thus, there is needed a stator current limiter, which operatesto change the setpoint of an associated automatic voltage regulator(AVR) (not shown) to reduce reactive power. In turbine ambient followingapplications it is desirable in accordance with the preferred embodimentto be able to bias the stator current limiter so that the limit changesas a function of coolant conditions. This is easily accomplished in thepreferred implementation because a signal already exists which can beused to modify the limiter to match the change in generator capabilityas coolant is changed.

Specifically, HTSMAXVA is a signal that is scaled for 5000 counts atrated coolant conditions and is compensated for changes in generatorterminal voltage. As coolant conditions change, this signal changes toreflect the change in generator capability. For instance, a decrease incold gas temperature from rated on an air cooled generator will resultin HTSMAXVA increasing based on the constant TMPSSB which is selectedbased on generator ratings for temperatures less than rated. TMPSSB isgiven by the equation

TMPSSB=250,000*((MAXVA@T _(—) LOW/GEN _(—) MVA){circumflex over ()}2−1)/(T _(—) RAT−T _(—) LOW)  (8)

Where MAXVA@T_LOW is the generator rating in MVA at the low temperaturerating of the generator, T_RAT is the rated temperature of the generatorin degrees Celsius and T_LOW is the low temperature rating of thegenerator in degrees Celsius.

TMPSSA is likewise given by the equation

TMPSSA=250000*(1−(MAXVA@HI _(—) TEMP/GEN _(—) MVA){circumflex over ()}2)/(T _(—) HIGH−T _(—) RATED)  (9)

Where MAXVA@HI_TEMP is the rating of the generator in MVA at the highestrated temperature and T_HIGH is the temperature in degrees Celsius atthe highest rated temperature.

Since a stator current limit is implemented with a fixed setpoint, thestator current feedback is preferably altered in order to providecompensation for temperature. This is accomplished by using the E1PFVblock, block 425 of FIG. 4, which includes the ability to divide theactual stator current feedback signal by HTSMAXVA/5000.

The preferred implementation generates an accurate representation of thegenerator capability for changes in coolant for both air-cooled andhydrogen cooled generators using constants H2SEXP for hydrogen cooledgenerators and TMPSSA and TMPSSB for air cooled generators. This changein generator capability can be used to modify the feedback variable thuspermitting fixed setpoint limiters to be used.

Thus, in a preferred implementation, in the case of the OEL whichinvolves field current, there is preferably provided a field currentregulator. The OEL becomes the upper part of the generator capabilitycurve (curve AB of FIG. 1). It is desirable that the exciter be able totransiently force more than rated field current through the generatorfield and thus there is provided an inverse time function to approximatethe capability of the generator field to accommodate the transient fieldcurrent. In accordance with the preferred embodiment, there is providedthe ability to change both the limiter and protection algorithms toreflect the fact that the generator field capability changes as afunction of the cooling conditions of the generator. Thus, there iscoordination between the limiter and protection functions even at thesenon-rated coolant conditions.

The UEL (Under Excited Limiter) performs a similar function but in anarea of the generator capability curve that is limited by end ironheating in the generator and steady state stability. This is basicallythe lower part of FIG. 1 (curves DC, DG or DK). The curve shown frompoint D to point C comprises the underexcited capability of thegenerator from zero real power to 0.95 per unit real power (point C is 1per unit KVA at 0.95 power factor in this case). This is an empiricalcurve. The curve shown from points C to B is a circle with center at 0watts and 0 vars and a radius of 1 per unit KVA. This curve isapproximated with the UEL portion of the preferred embodiment using astraight line with fixed slope from point N through point Q. Thestraight line with fixed slope approximation is used rather than theactual circle because the slope of the circle at point Q has infinitegain (delta Q/delta P) which would result in infinite gain in the UELregulator (not shown). A high value select circuit, block 660, is usedto take the higher of the curve shown from D to C or the straight linesegment from N to Q. There is no corresponding protection function forthe UEL that requires coolant compensation but the instantimplementation provides a very good approximation to the variation ofthe generator capability curve with coolant conditions.

Similar to the UEL, a stator current limit function is used to restrictoperation of the generator in the area of the capability curve limitedby generator stator current. This is the constant KVA area shown as theright hand part of the capability curve of FIG. 1 (curve BC) and is thesame circle used by the UEL. In this case, a circle characteristic isused because the stator current limit is a set point, very lowbandwidth, controller. The disclosed algorithms provide very accurateimplementation of the circular generator capability curves as a functionof generator cooling conditions.

While the foregoing description includes numerous details and specifics,it is to be understood that these are provided for purposes ofexplanation only, and are not intended to limit the scope of theinvention. Those of ordinary skill in the art will easily be able tomake numerous modifications to the exemplary embodiments described abovewithout departing from the scope of the invention, as defined by thefollowing claims and their legal equivalents.

What is claimed is:
 1. An apparatus for controlling an electricalgenerator, comprising: a summing block having as inputs a rating valueof said generator and a value indicative of a cooling condition of saidgenerator; a circuit for calculating, based on an output of said summingblock, a compensation value for limits of predetermined portions of acapability curve of said generator; and at least one of a ratio blockand a setpoint block receiving said compensation value and outputting afeedback signal to a fixed limiter function of said generator.
 2. Theapparatus of claim 1, further comprising at least one of a temperaturesensor and pressure sensor for monitoring said cooling condition.
 3. Theapparatus of claim 2, further comprising a low pass filtering connectedbetween said sensor and said summing block.
 4. The apparatus of claim 1,further comprising a switch for automatically selecting one of effectingcalculations based on generator coolant temperature and coolantpressure.
 5. The apparatus of claim 1, wherein said circuit comprisesmeans for calculating a result based on a log—log relationship betweengenerator field current and coolant pressure.
 6. The apparatus of claim1, wherein said circuit comprises a means for calculating a result basedon a quadratic relationship between generator field current and coolanttemperature.
 7. The apparatus of claim 1, further comprising connectionto inputs of at least one of an underexcited fixed limiter function, anoverexcited fixed limiter function, an automatic voltage regulator andan inverse time limit and protection function.
 8. The apparatus of claim1, further comprising a scaling block connected upstream from summingblock.
 9. The apparatus of claim 1, wherein said apparatus controls theamount of at least one of field current and stator current in saidgenerator.
 10. The apparatus of claim 1, further comprising anormalizing circuit for normalizing said compensation value with respectto generator terminal voltage.
 11. The apparatus of claim 1, furthercomprising a lookup table for emulating at least one portion of saidcapability curve.
 12. The apparatus of claim 1, wherein the apparatuscomprises a part of an exciter of said generator.
 13. A method ofcontrolling an electrical generator, comprising the steps of:determining the rating of said generator; monitoring a cooling conditionof said generator; calculating, based on said rating and coolingconditions, a compensation value for limits of predetermined portions ofa capability curve of said generator; and modifying a feedback signal toa fixed limiter function in accordance with said compensation value. 14.The method of claim 13, further comprising monitoring one of atemperature sensor and pressure sensor to monitor said coolingcondition.
 15. The method of claim 14, further comprising low passfiltering an output of said sensor.
 16. The method of claim 13, furthercomprising summing a generator rating value with a value indicative ofsaid cooling condition and employing the resulting sum as a multiplierto effect said modifying step.
 17. The method of claim 13, furthercomprising automatically selecting one of effecting said calculatingstep based on generator coolant temperature and coolant pressure. 18.The method of claim 13, wherein said calculating step comprises acalculation based on a log—log relationship between generator fieldcurrent and coolant pressure.
 19. The method of claim 13, wherein saidcalculating step comprises a calculation based on a quadraticrelationship between generator field current and coolant temperature.20. The method of claim 13, further comprising varying a feedback signalto at least one of an underexcited fixed limiter function, anoverexcited fixed limiter function, an automatic voltage regulator andan inverse time limit and protection function.
 21. The method of claim13, further comprising scaling a value indicative of said coolingcondition.
 22. The method of claim 13, further comprising controllingthe amount of at least one of field current and stator current in saidgenerator.
 23. The method of claim 13, further comprising normalizingwith respect to generator terminal voltage said compensation value. 24.The method of claim 13, further comprising electronically emulating atleast one portion of said capability curve.
 25. The method of claim 13,further comprising providing transient-forcing capability for saidgenerator by coordinating said limiter function with a protectionfunction of said generator.